Melting and spinning device and melting and spinning method

ABSTRACT

The present invention is relative to a melt spinning apparatus for melt spinning a biodegradable polymer material. The biodegradable polymer material is melted by a melt mechanism ( 4 ) including a screw ( 16 ) which is mounted in a vertically mounted cylinder ( 8 ) coaxially with the cylinder ( 8 ) and which is rotationally driven by a rotational driving mechanism ( 7 ). The screw includes at least one turn of a helical groove ( 17 ) on its peripheral surface. The biodegradable polymer material being melted in this way is discharged in the vertical direction from a discharge opening of a nozzle ( 10 ) mounted coaxially on the cylinder ( 8 ).

TECHNICAL FIELD

[0001] This invention relates to a method and an apparatus for melt spinning a medical material implanted in a living body, for example, a strand of a biodegradable polymer material which forms a stent implanted in the vascular vessel of a living body.

BACKGROUND ART

[0002] When a stenosed lesion has occurred in the vascular vessel of a living body, in particular the blood vessel, such as arterial vessels, the percutaneous transluminal angioplasty (PTA) is performed by inserting a balloon mounted in the vicinity of the distal end of a catheter into the stenosed lesion and inflating this balloon to expand the stenosed lesion to keep the blood flowing.

[0003] Meanwhile, it is known that, even if PTA is applied, there is a high probability that re-stenosis is liable to occur at the once-stenosed portion.

[0004] In order to prevent this re-stenosis from occurring, the conventional practice is to implant a tubular stent on the site where the PTA has been performed. This stent is inserted in a contracted state into the blood vessel, subsequently dilated and implanted in this state in the blood vessel to support the blood vessel from its inside to prevent re-stenosis from occurring in the blood vessel. As this sort of the stent, the metallic stent formed of such as stainless steel or a Ti—Ni based alloy, is now in use.

[0005] Meanwhile, the principal objective in implanting a stent in the blood vessel in PTA is to prevent acute coronary occlusion and to decrease the frequency of re-stenosis. It has been reported that, since the acute coronary occlusion and re-stenosis are the phenomenon which occurs during a predetermined time period, so that only transient therapy is needed. Consequently, the stent is required to maintain the function of supporting the blood vessel from inside for a predetermined time period, while it is more desirable that the stent is not left in the living body as a foreign substance.

[0006] If the metallic stent is implanted in the blood vessel, it is left permanently, so that, when re-stenosis occurs on the stent site, the stent frequently proves an obstruction to the operation of re-angioplasty. Moreover, the operation of coronary-artery bypass graft is difficult to perform on the site of implanted stent. Thus, implanting the permanently persisting metallic stent offers various inconveniences to re-treatment.

[0007] In order to overcome the problem inherent in the metallic stent, such a stent formed of a biodegradable polymer material has been proposed which i s degraded after a lapse of a predetermined time, from the time it is implanted in e.g., the blood vessel of the living body to be then disappear by being absorbed in the living tissue (JP Patent No. 2842943; JP Laying-Open Patent Publication H-11-57018).

[0008] The present inventors have proposed a stent comprised of a knitting obtained on knitting a strand of the biodegradable polymer material into a tubular form (JP Patent 2842943), and a stent obtained by bending a strand of a biodegradable polymer in a zigzag shape and wrapping it in a tubular form under a non-woven non-knitted condition.

[0009] With the use of the strand formed of a biodegradable polymer, it is possible to font a stent which exhibits mechanical characteristics sufficient to support the vessel in the inflated state for certain time duration and disappears after lapse of the preset time.

[0010] Since the stent formed of the strand of the biodegradable polymer can readily be flexed and deformed, it can readily be delivered through the sinuous blood vessel so as to be implanted on the target site.

[0011] It should be noted that the biodegradable polymer material, as a high molecular material, differs in its degradation and absorption characteristics, and hence in its mechanical properties, depending on the molecular weight. For example, the molecular weight of the biodegradable polymer material, such as polylactic acid (PLA), is lowered by being melted and thermally decomposed. The degree that the molecular weight is lowered changes depending on the degree of thermal decomposition. Thus, if the melt spinning heating time of the same biodegradable polymer material is non-uniform, then the average molecular weight of the spun strand becomes non-uniform. If the strand is non-uniform in its average molecular weight, its degradation and absorption characteristics or mechanical properties undergo localized variations.

[0012] If, with non-uniform average molecular weight, a stent is formed and implanted in the vascular vessel, such as a blood vessel, the stent in its entirety cannot be degraded or absorbed evenly. Moreover, there is a fear that the stent formed using this sort of the strand cannot support the inner wall of the vascular vessel, such as blood vessels, with a uniform force, because the strand itself exhibits strength variations.

DISCLOSURE OF THE INVENTION

[0013] It is therefore an object of the present invention to provide a method and an apparatus for spinning the strand, whereby it is possible to spin the strand having uniform mechanical properties and uniform degradation and absorption characteristics free from strength variations, that is, it is possible to spin the strand having a uniform average molecular weight and making it a suitable construction material for a biodegradable stent.

[0014] The present invention provides a melt spinning apparatus for melt spinning a strand of a biodegradable polymer material, forming a stent implanted in a living body. This comprises a vertically mounted cylinder, supplied with the biodegradable polymer material, a screw mounted in the cylinder coaxially, rotationally driven by a rotational driving unit and having at least one turn of a helical groove on its peripheral surface, and a nozzle mounted to the distal end of the cylinder and having a discharge opening coaxially with the cylinder. The biodegradable polymer material supplied into the cylinder and melted by rotation of the screw is emitted vertically from a discharge opening in the nozzle for spinning the strand.

[0015] With the present melt spinning apparatus, the molten biodegradable polymer material is fed by a screw in the vertical direction and emitted from a nozzle for spinning a strand, so that the strand has a uniform molecular weight distribution is spun as stagnation or non-uniform eddying currents of the biodegradable polymer material melted in the cylinder or the nozzle may be prevented from being produced.

[0016] Moreover, with the present melt spinning apparatus, there is provided a plural number of heating units placed in juxtaposition along the axial direction of the cylinder, on the outer sides of the cylinder forming the melt mechanism for melting the biodegradable polymer material, for controlling the molten state of the biodegradable polymer material injected into the cylinder. The heating units are able to perform temperature control independently of one another.

[0017] The nozzle for discharging the molten biodegradable polymer material is kept at a constant temperature by the heating units. By controlling the nozzle temperature, the temperature of the molten biodegradable polymer material discharged from the nozzle can be made constant.

[0018] According to the present invention, the biodegradable polymer material is melted by a melt mechanism including a screw which is provided in a vertically mounted cylinder coaxially and on the peripheral surface of which at least one turn of the helical groove is formed. The screw is rotated by a rotational driving mechanism. The molten biodegradable polymer material is emitted in the vertical direction through a discharge opening in a nozzle provided coaxially with the cylinder for spinning the strand.

[0019] With the melt spinning method of the present invention, a spun filament having a uniform molecular weight distribution may be produced.

[0020] Other objects, features and advantages of the present invention will become more apparent from reading the embodiments of the present invention as shown in the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021]FIG. 1 is a side view showing a melt spinning apparatus according to the present invention.

[0022]FIG. 2 is a cross-sectional view showing the cylinder and the screw of a melting mechanism.

[0023]FIG. 3 is a plan view showing a flow resistance plate mounted to the distal end of the cylinder.

[0024]FIG. 4 is a cross-sectional view showing a discharging unit at the distal end of the melt mechanism.

[0025]FIG. 5 is a cross-sectional view showing a supply unit for supplying a polymer material to the melting mechanism.

[0026]FIG. 6 is a side view showing the screw that is placed in the cylinder forming the melting mechanism.

[0027]FIG. 7 is a side view showing a supply controlling mechanism which is placed between the melting mechanism and the discharging unit.

[0028]FIG. 8 is a cross-sectional view showing a set of gears forming the melt mechanism.

BEST MODE FOR CARRYING OUT THE INVENTION

[0029] A melt spinning apparatus and method for melt spinning the biodegradable polymer material using the melt spinning apparatus are now explained in detail.

[0030] The melt spinning apparatus according to the present invention is of a vertical type in which a melt spinning unit is mounted vertically, as shown in FIG. 1.

[0031] The melt spinning apparatus, shown in FIG. 1, includes a base plate 2, mounted horizontally on a mounting surface, and the melt spinning unit 1 is supported by member 3 a by a support pillar 3 mounted upright on the base plate 2. The melt spinning unit 1 includes: a melt mechanism 4 which is supported by and parallel to the upstanding support pillar 3, a discharging unit 5 for discharging the polymer material melted by the melt mechanism 4, a supplying unit 6 for supplying the polymer material to the melt mechanism 4, and a rotational driving mechanism 7 for rotationally driving a screw 16 forming the melt mechanism 4.

[0032] The melt mechanism 4 which is in the melt spinning unit 1, includes a cylinder 8, as shown in FIG. 2. The screw 16 is provided within and coaxially of the cylinder 8. The screw 16 pressurizes the polymer material, injected into the cylinder 8 and extrudes the pressurized material towards the distal end of the cylinder 8 while melting it.

[0033] On the outer periphery of the cylinder 8, there is a plural number of heating units 9 in juxtaposition along the axial direction of the cylinder 8. These heating units 9 are controlled independently of one another to enable the multi-stage control of the temperature axially of the cylinder 8.

[0034] On the distal end of the cylinder 8, there is mounted a connecting member 21 for connecting the discharging unit 5 to the cylinder. The discharging unit 5 is adapted for discharging the melted polymer material. The connecting member 21 is ring-shaped and has at its center portion a flow resistance plate 22 including a plural number of through-holes 22 a matching the axial direction of the screw 16, as shown in FIG. 3. The molten polymer material, supplied from the distal end of the cylinder 8, by rotating the screw 16, is pressurized by the flow resistance when traversing the flow resistance plate 22. The pressurized polymer material is discharged towards the discharging unit 5 from the distal end of the cylinder 8.

[0035] The diameter or the number of the through-holes 22 a provided on the flow resistance plate 22 is changed depending on the amount or supply rate of the molten polymer material supplied from the distal end of cylinder 8, on rotation of screw 16, or on the viscous resistance of the polymer material.

[0036] The flow resistance plate 22 may be of any shape provided that it affords the flow resistance to the polymer material supplied melted from the distal end of the cylinder 8 on rotation of the screw 16 to pressurize the polymer material.

[0037] The discharging unit 5, mounted by the connecting member 21 at the distal end of the cylinder 8, includes a sprue bush 11, connected to the distal end of cylinder 8, and a nozzle 10 mounted to the distal end of the sprue bush 11, as shown in FIG. 4. The nozzle 10 is secured to the distal end of the sprue bush 11 by a mounting member 12. Meanwhile, the nozzle 10 and the mounting member 12 can be the same.

[0038] The sprue bush 11, which is part of the discharging unit 5, supplies the molten polymer material from cylinder 8, to the nozzle 10 in a stable state at a constant rate of amount per unit time. A flow passage 11 a is in the center co-axially of the cylinder 8, as shown in FIG. 4. That is, the flow passage 11 a and the cylinder 8 are placed vertically with a common axis P1. The flow passage 11 a is tapered moderately from the vertically placed cylinder 8 towards the nozzle 10 so that the polymer material supplied melted from the cylinder 8 may be supplied in succession by predetermined amounts per unit time to the nozzle 10 without producing stagnation or eddying currents.

[0039] The nozzle 10 includes a discharge opening 10 a for discharging the polymer material supplied melted from the sprue bush 11, as shown in FIG. 4. The discharge opening 10 a operates for controlling the diameter of the spun strand and is formed by an optimum diameter depending on the thickness of the spun strand. The discharge opening 10 a is also formed to be coaxial with the flow passage 11 a. That is, the discharge opening 10 a and the flow passage 11 a are set upright coaxially as the cylinder 8.

[0040] Meanwhile, plural different nozzles with different diameters R₁ at the discharge opening 10 a may be provided and exchanged from time to time to spin the strand with different thicknesses.

[0041] On the outer periphery of the discharging unit 5, there is a heating unit 13 for controlling the temperature of the discharging unit 5. This heating unit 13 controls the temperature of the discharging unit 5 to control the temperature of the polymer material discharged from the nozzle 10.

[0042] The supplying unit 6 for supplying the molten polymer material to the melt mechanism 4 includes a hopper 14 for loading the polymer material into the cylinder 8 and a mounting unit 6 a for mounting the unit 6 to the cylinder 8, as shown in FIG. 5. On the outer periphery of the mounting unit 6 a, there is provided a temperature controller 15 for controlling the temperature of the supplying unit 6. This temperature controller 15 keeps the polymer material, loaded into the hopper 14, at a constant temperature, and is comprised of a heating/cooling means.

[0043] The melt mechanism 4 is explained more specifically. Referring to FIG. 6, the melt mechanism 4 includes the screw 16, having a helically extending groove 17 on its peripheral surface, is mounted coaxially within the cylinder 8. The screw 16 is rotationally driven by the rotational driving mechanism 7 at the proximal end where the screw is connected. When the screw 16 is driven rotationally, the polymer material, loaded into the cylinder 8 and melted by the heating units 9, is fed to the distal end of the cylinder 8.

[0044] Meanwhile, the helical groove of the screw used in the routine melt spinning apparatus, is formed to a pitch subsequently equivalent to the screw diameter. The helical groove 17 of the screw 16 used in the melt spinning apparatus according to the present invention has a pitch Tp equal to one-half the diameter Sr of the screw 16. By forming the helical groove 17 in this manner, the dwelling time of the injected polymer material in the cylinder 8 can be protracted, such that melting can be achieved reliably by sufficient heating in the heating unit 9 even though the screw 16 is of a reduced length. By employing the screw 16, the screw length may be reduced, as a result of which the melt mechanism 4 including the cylinder 8 can be reduced in size.

[0045] It should be noted that the melt spinning apparatus of the present invention may be provided with a supply controlling mechanism 18, between the melt mechanism 4 and the discharging unit 5, for controlling the supply quantity of the polymer material in molten state, which is supplied to the discharging unit 5. This supply controlling mechanism 18 may be configured as shown for example in FIG. 7. The supply controlling mechanism 18, shown in FIG. 7, includes a pressure detection means 19 for measuring the pressure of the polymer material extruded from the melt mechanism 4 and circulated in the molten state through a flow passage 18 a, and a set of gears 20 for feeding the melted polymer material to the discharging unit 5. This supply controlling mechanism 18 detects the pressure of the polymer material flowing through the flow passage 18 a by the pressure detection means 19. The rotation of the set of gears 20 is controlled by this detection output to keep the pressure of the polymer material flowing through the flow passage 18 a constant. By controlling the pressure of the polymer material flowing through the flow passage 18 a at a constant magnitude, a preset constant quantity of the polymer material can be supplied to the discharging unit 5.

[0046] A heating unit 23 is on the outer periphery of the portion of the supply controlling mechanism 18, which controls the temperature of the polymer material flowing through the flow passage 18 a at a preset temperature.

[0047] The melt spinning method employed by the melt spinning apparatus of the present invention is now explained.

[0048] The present invention melt-spins the strand, formed of a biodegradable polymer material used for forming a stent implanted in the living body. The melt spun polymer material, used herein, is the biodegradable polymer material. The biodegradable polymer material may be enumerated by polylactic acid (PLA), polyglycolic acid (PGA), polyglactin (polyglycolic acid-polylactic acid copolymer), polydioxanone, polyglyconate (trimethylene carbonate-glycoid copolymer) and a polylactic acid-ε-caprolactone copolymer.

[0049] For spinning the polymer material, a pellet-like polymer material Pp is charged into a hopper 14 of the supplying unit 6. The polymer material, loaded into the hopper 14, is supplied to the cylinder 8 of the melt mechanism 4.

[0050] In order for the polymer material, loaded into the hopper 14, to be quickly supplied into the helical groove 17 formed in the screw 16 rotating in the cylinder 8, the polymer material needs to be in solid state. That is, the polymer material, supplied into the cylinder 8, needs to be controlled to a temperature not higher than its melting point (Tm) or softening point. For shortening the melt time in the melt mechanism 4, the polymer material, supplied to the cylinder 8, needs to be melted immediately. Thus, the temperature controller 15, provided in the supplying unit 6, sets the temperature of the polymer material, charged into the hopper 14, to a temperature at which the polymer material can be melted immediately as it maintains its solid state.

[0051] The polymer material, supplied into the cylinder 8 through the hopper 14, is introduced into the helical groove 17 of the screw 16, rotated by the rotational driving mechanism 7, so as to be extruded towards the distal end of the cylinder 8, as it is heated by the heating units 9 provided on the outer periphery of the cylinder 8. As the polymer material is extruded, the temperature of the polymer material is controlled to be lower than its thermal decomposition temperature so as not to cause transmutation of the polymer material. The polymer material, thus controlled to a temperature not higher than its thermal decomposition temperature, is positively extruded from the distal end of the cylinder 8 as it is kept in molten state without undergoing transmutation.

[0052] The polymer material, extruded at the distal end of the cylinder 8 while in its molten state, is afforded with flow resistance by the flow resistance plate 22, in such a manner that it is evenly pressurized by the through-holes 22 a. The polymer materia, thus pressurized, is supplied to the discharging unit 5.

[0053] Since the through-holes 22 a formed in the flow resistance plate 22 are oriented vertically in order not to produce stagnation or eddying currents in the polymer material, the polymer material can be supplied to the discharging unit 5 as the molecular weight distribution is maintained to be constant.

[0054] If the melt spinning apparatus has the supply controlling mechanism 18 between the melt mechanism 4 and the discharging unit 5, the molten polymer material, extruded from the cylinder 8 of the melt mechanism 4, is maintained at a constant pressure by the supply controlling mechanism 18, so that it is controlled in flow rate at the discharging unit 5 and is reliably supplied to the discharging unit 5 at a constant flow rate.

[0055] The polymer material supplied to the supply controlling mechanism 18 is heated by the heating unit 23 provided on the outer periphery of the supply controlling mechanism 18 and hence is delivered to the discharging unit 5, reliably in its molten state. The heating unit 23 maintains the heating temperature at less than the thermal decomposition temperature so as not to cause transmutation of the polymer material.

[0056] The molten polymer material, delivered from the melt mechanism 4 or the supply controlling mechanism 18 to the discharging unit 5, is heated in the sprue bush 11 by the heating unit 23 to a temperature less than the thermal decomposition temperature. Since the flow path of the discharging unit 5 from the sprue bush 11 to the nozzle 10 is oriented vertically, the polymer material flowing therein is not subjected to stagnation or eddying currents. Since the polymer material, maintained in its molten state, may thus be supplied to the nozzle 10 through the vertical flow path, it can be discharged at the nozzle 10 as it maintained in the state of uniform molecular weight distribution, and hence the strand of the polymer material can be spun with uniform molecular weight distribution.

[0057] It should be noted that a monofilament strand may be spun because the sole discharge opening 10 a is formed through the nozzle 10 for extending in the vertical direction.

[0058] Industrial Applicability

[0059] With the melt spinning method and apparatus of the present invention, it is possible to prevent stagnation or nonuniform eddying currents of the biodegradable polymer material in order to spin the strand into a uniform average molecular weight. That is, the strand of the biodegradable polymer material may be spun which is uniform mechanical properties and degradation and absorption characteristics. This spun strand can be used to the utmost advantage for forming a stent inserted into the vascular vessel of the living body. 

1. A melt spinning apparatus for melt spinning a strand of a biodegradable polymer material, forming a stent implanted in a living body, comprising: a vertically mounted cylinder, supplied with said biodegradable polymer material; a screw mounted in said cylinder coaxially therewith, said screw being rotationally driven by a rotational driving unit and having at least one turn of a helical groove on its peripheral surface; and a nozzle mounted to the distal end of said cylinder and having a discharge opening coaxially with said cylinder; said biodegradable polymer material supplied into said cylinder and melted by rotation of said screw being emitted vertically from a discharge opening in said nozzle for spinning the strand.
 2. The melt spinning apparatus according to claim 1 wherein a sole discharge opening is provided in said nozzle for extending in the vertical direction for spinning a filament.
 3. The melt spinning apparatus according to claim 1 further comprising a plurality of heating units in juxtaposition along the axial direction of said cylinder, with the heating temperature of said heating units being controlled independently of one another.
 4. The melt spinning apparatus according to claim 2 further comprising heating units on the outer periphery of said nozzle for controlling the nozzle temperature.
 5. The melt spinning apparatus according to claim 1 wherein the helical groove formed on the peripheral surface of said screw is formed to a pitch smaller than one-half the screw diameter.
 6. The melt spinning apparatus according to claim 1 further comprising a supplying unit for loading a biodegradable polymer material into said cylinder and temperature controlling mechanism for controlling the temperature of the biodegradable polymer material charged into said supplying unit.
 7. A method for melt spinning a strand of a biodegradable polymer material, forming a stent implanted in a living body, comprising: melting the biodegradable polymer material by a melt mechanism including a screw which is provided in a vertically mounted cylinder coaxially therewith and on the peripheral surface of which at least one turn of the spiral groove is formed, said screw being rotated by a rotational driving mechanism; the molten biodegradable polymer material being emitted in the vertical direction through a discharge opening in a nozzle provided coaxially with said cylinder for spinning the strand.
 8. The melt spinning method according to claim 7 wherein the molten biodegradable polymer material is emitted in the vertical direction from a sole discharge opening provided in said nozzle for spinning into a filament.
 9. The melt spinning method according to claim 7 wherein the biodegradable polymer material is heated in a supplying unit to a temperature lower than the melting temperature and is loaded into said melt mechanism for melting.
 10. The melt spinning method according to claim 7 wherein said biodegradable polymer material is one of polylactic acid (PLA), polyglycolic acid (PGA), polyglactin (a polyglycolic acid-polylactic acid copolymer), polydioxanone, polyglyconate (a trimethylene carbonate-glycoid copolymer) and a polylactic acid-ε-caprolactone copolymer. 